1. Field of the Invention
The invention relates to a right and left driving force distribution unit in a four-wheel drive vehicle.
2. Description of Related Art
In a four-wheel drive vehicle, one of a two-wheel drive mode and a four-wheel (all-wheel) drive mode is selected. In the two-wheel drive mode, driving force is transmitted from a driving source to front or rear right and left main drive wheels. In the four-wheel (all-wheel) drive mode, driving force is transmitted from the driving source to the main drive wheels and right and left auxiliary drive wheels. Other than power transmission members that transmit driving force from the driving source to the front or rear right and left main drive wheels in the two-wheel drive mode, there is another power transmission member that transmits driving force to the auxiliary drive wheels in the four-wheel drive mode and that is rotated in the two-wheel drive mode. As a result, part of power is consumed because of rotational resistance that is generated by rotation of the other power transmission member, which may lead to undesirable influence on fuel economy of the vehicle.
In contrast, there has been suggested a four-wheel drive vehicle including a right and left driving force distribution unit equipped with a disconnect mechanism. In the two-wheel drive mode, the disconnect mechanism is used to disconnect the other power transmission member (propeller shaft) from drive wheels that are used in the four-wheel drive mode. The other power transmission member transmits driving force for a four-wheel drive mode. This is, for example, a four-wheel drive vehicle shown in FIG. 1 of U.S. Pat. No. 8,313,407.
A rear wheel-side right and left driving force distribution unit shown in FIG. 1 of U.S. Pat. No. 8,313,407 includes a differential gear unit in order to distribute driving force to right and left drive wheels. Instead of the differential gear unit, it is conceivable that a pair of friction clutches for right and left distributing driving force are respectively provided in right and left axles so as to be in series with the disconnect mechanism as shown in, for example, FIG. 7 of U.S. Pat. No. 8,313,407. With this configuration, a differential gear case fixed to a ring gear of the differential gear unit, a pair of side gears opposed to each other inside the differential gear case, pinions rotatably supported by a pinion pin fixed to the differential gear case, and the like, are unnecessary. The diameter of the ring gear is reduced.